deps: quick-xml DoS advisories (RUSTSEC-2026-0195/-0194) unfixable by version bump — alternative profiling-dependency options

Tracking follow-up from a security-dependency audit of the workspace (a branch with the fixable bump — crossbeam-epoch RUSTSEC-2026-0204 — plus documented ignores is ready and will arrive as a PR).

The advisories: RUSTSEC-2026-0195 / RUSTSEC-2026-0194 (quick-xml 0.26.0, DoS-class, severity 7.5) flag on cargo audit.

Why a version bump can't fix it today (verified by live dependency resolution, not just lockfile reading):

quick-xml 0.26.0  <-  inferno 0.11.21  <-  pprof 0.13  <-  ruvector-bench (optional, behind the `profiling` feature)

• pprof 0.15.0 (its newest release) still requires inferno = "^0.11"
• inferno 0.11.21 (its newest 0.11 patch) still requires quick-xml = "^0.26"

So the vulnerable pin is upstream of this repo on every current release. Current quick-xml releases are unaffected; the chain just can't reach them yet.

Mitigating context: the exposure is dev-tooling only — pprof is optional and only enabled via ruvector-bench's profiling feature, so no default or production build ships the vulnerable code path.

Alternatives worth considering (any one of these clears the advisory rather than ignoring it):

1. Drop in-process flamegraph rendering: use pprof's protobuf output feature instead of flamegraph, and render externally (go tool pprof, speedscope). Removes inferno → quick-xml entirely; keeps criterion integration.
2. External profiler workflow: samply or cargo flamegraph as documented dev tools outside the dependency graph — zero workspace deps, same artifact.
3. Upstream nudge: an issue on inferno asking for a quick-xml major bump would fix the whole ecosystem's chain; pprof would follow.
4. Status quo + documented ignore (what the audit branch does meanwhile): keeps cargo audit/cargo deny green with an explicit rationale entry rather than a silent red.

Happy to send the PR for option 1 or 2 if either fits how you actually use the profiling feature.

Start by reproducing the dependency chain with cargo audit and checking the profiling feature in ruvector-bench: pprof 0.13, inferno 0.11.21, and quick-xml 0.26.0. Compare the proposed protobuf/external-profiler options and the documented-ignore fallback; done means one chosen approach clears the advisory or records an explicit rationale while preserving the intended profiling workflow.